Hollow fiber membrane for the treatment of waste lubricants and method for its production

ABSTRACT

The present invention proposes a new formulation for the making of a cellulose acetate hollow fiber membrane for ultrafiltration with high water permeability, capable of oil and water separation with minimal energy consumption and low fouling tendencies.

FIELD OF THE INVENTION

The present invention relates to the purification of water contaminatedwith oily lubricants using cellulose acetate hollow fiber membranes.

BACKGROUND OF THE INVENTION

Lubricant fluids are widely used in machining processes like metalfinishing, metal working and also in the electronics industries. Theyare used for a variety of reasons such as improving equipment life,reducing work piece thermal deformation, improving surface finish andflushing away impurities from the work zone.

There are various categories of such lubricants and one of the mostpopular is the soluble oil fluids. The oil concentrate of such oilfluids consists of mineral oils and some emulsifiers. They are used in adiluted form with water having an oil concentration of about 3-10%.Together they form a stable emulsion when mixed with water. Theresulting oil-in-water emulsion typically has a milky white appearance.After extended periods of use, the emulsion usually becomes inefficientthrough progressive degradation or contamination and requiresreplacement. The waste lubricant fluid has to be treated to localenvironmental sewage standards before it can be disposed of properly.Current practices of waste lubrication fluid treatment involvesessentially two steps: 1) Solid-liquid separation and then 2)Liquid-liquid separation.

Solid-liquid separation is to remove solid contaminants like work piecedebris from the waste fluid before going to the second step. Currentmethods basically involve some form of filtration or centrifugal actionto separate the solid contaminants from the liquids. Solid contaminantscan also be separated by allowing the solids to settle to the bottom ofa container.

Liquid-liquid separation is more complex. The oil-in-water emulsion hasto be broken into its separate oil and water components before disposal.This is often done using chemicals to break the emulsion. However, thismethod results in more chemicals being present in the water. The oillayer is then skimmed off to separate the oil from the water. The waterhas to be further treated before discarding while the recovered oil isoften reused as fuels or simply incinerated. Another method used is tosubject the liquid to centrifugal action to separate the oil from water.

As can be seen, the treatment of such oil-in-water emulsions from wastelubricants is not a simple matter. The equipment, space and costsinvolved are generally quite high. Merely separating the oil from thewater is not enough. The water must meet the environmental standards oflocal authorities before it can be discharged. Most current methods donot provide a simple solution to separating the oil from the water andtreating the water at the same time. In addition, some of the oildroplets formed in such emulsions are so fine that they cannot be easilyseparated by the above mentioned conventional methods.

A known proposed alternative is the use of ultrafiltration membranes toseparate the oil from the water in such emulsions. This method also hasthe distinct advantage of reducing Chemical Oxygen Demand (COD) levelswhich is a criteria in waste water disposal. However, present use ofcommercially available tubular or hollow fiber membranes for thetreatment of such waste lubricant fluids causes serious fouling problemsdue to the hydrophobic characteristics of the membranes. Furthermore,the equipment is expensive and consumes large amounts of energy. Thelife of the membranes could be greatly affected due to the seriousfouling problem associated with currently available membranes.

The present invention proposes a new formulation for the making of acellulose acetate hollow fiber membrane with high water permeability,capable of oil and water separation with low fouling tendencies.

OBJECTIVE OF THE PRESENT INVENTION

To provide a new cellulose acetate ultrafiltration hollow fiber membranecapable of separating oils from waste lubricant fluids with thefollowing characteristics: low fouling by oil, high water permeabilityand high COD and oil removal. To provide a method of producing such amembrane that has all the above listed characteristics.

SUMMARY OF THE INVENTION

The objectives of the invention are achieved by producing a hollow fibermembrane from cellulose acetate. The characteristics of the membraneare: a molecular weight cut-off (MWCO) of 5,000 to 30,000, a pure waterpermeability of 100 to 300 L/m².h.bar and a low fouling tendency by theretentate(oil). The method of manufacture of the hollow fibers has alsobeen made simpler to reduce costs and simplify production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an oil droplet size distribution chart of the emulsion.

FIG. 2 is a chart showing the changes in permeation flux of themembranes and COD levels in permeate over a prolonged 70 hourultrafiltration run.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Cellulose acetate was chosen as the membrane material because of itshigh hydrophilicity (i.e. having an affinity for water) which favors thereduction of fouling tendencies of the resulting membranes. Its uniquecharacteristics are known to be suitable for the production of membraneswith high water permeability for the treatment of oily wastewater.

Commercially available cellulose acetate was used as the presentmembrane material. An organic solvent was selected to dissolve thecellulose acetate. In addition, non-solvent additives which are alsoknown as modification agents are also required. Together thesecomponents form the doping solution.

A tube-in-orifice spinneret is used to form the hollow fibers via thephase inversion technique or sometimes referred to as immersionprecipitation. In this method, an external coagulant or a precipitationbath and an internal coagulant or bore liquid are required to form thehollow fibers.

In a preferred embodiment, the doping solution contains 15-25 wt. % ofcellulose acetate polymer, 60-81 wt. % of organic solvent and 4-15 wt. %of non-solvent additives or modification agents. The organic solvent isN-methyl-2-pyrollidone (NMP). The non-solvent additives or modificationagents comprise of polyvinylpyrrolidone(PVP), inorganic or organicacids, inorganic salts or mixtures of all or some of the mentionedcompounds. The external coagulant or precipitation bath used is simplyfresh water. The internal coagulant or bore liquid used is either wateror a mixture of water and NMP, where the mixture has a NMP compositionof 20-80 wt. % in water. Other well known organic solvents that may beused are: dimethylacetamide, acetone, dimethylsulfoxide,dimethylformamide and dioxan.

The required amount of solvent and cellulose acetate polymer were placedin a reaction flask. A stirrer was set at a speed of about 500 rpm so asto ensure that all the cellulose acetate polymer pellets were dissolved.The non-solvent additives or modification agents were then introducedinto the flask. Stirring is continued until all the cellulose acetatepellets and additives were completely dissolved. To remove any gasbubbles in the doping solution, it was vacuum degassed at roomtemperature.

The spinning solution was further allowed to stand in a stainless steeltank for twelve hours to ensure proper degassing prior to spinning. Thehollow fibers were formed via phase inversion technique using atube-in-orifice spinneret. The doping solution was extruded at acontrolled rate of about 3.0 to 5.0 ml/min while the internal coagulantsor bore liquid was introduced at a similar rate forming a contiguousinterior cavity of the hollow fiber. The extruded hollow fibers werethen passed into an external coagulant or precipitation bath of freshwater to complete the solidification process. The extruded hollow fibersmay be exposed to air for a gap of between 0-50 cm from the spinneretbefore reaching the precipitation bath. Any residual solvents andnon-solvent additives in the solidified hollow fibers were removed byfresh water leaching in a storage tank for at least 48 hours prior touse. Furthermore, the hollow fibers are stored in fresh water to preventdrying up of the hollow fibers which would lead to the collapse of themembrane pores.

It is important to note that the entire process of preparation of thehollow fibers was done at a temperature of between 10° C. to 30° C.

Experimental/Preliminary Studies

The resulting cellulose acetate hollow fiber membranes exhibit a MWCO ofabout 5,000 to 30,000 daltons and a pure water permeability of 100 to300 Um².h.bar. The physical attributes of the hollow fibers are: aninternal diameter of about 1,000 to 1,500 microns and a wall thicknessof about 200 to 500 microns.

A cross-flow ultrafiltration unit was setup for tests at roomtemperature. It was fitted with an ultrafiltration cellulose acetatehollow fiber membrane module with a filtration area of 0.005 m². Thefeed liquid pumped into the membrane module was a waste lubricant fluidfrom a precious metal fine extrusion process. The emulsion of this wastelubricant fluid contains oil content of about 10%. The oil droplet sizedistribution is shown in FIG. 1. and is observed that the oil dropletsin the emulsion are extremely small and are mainly under 1 micron.

The waste lubricant fluid was first filtered using a simple media filterto remove large solid contaminant particles. Prior to theultrafiltration, the waste lubricant fluid COD was measured and found tobe about 13,000 mg/L. During the ultrafiltration, trans-membranepressure or the feed pump pressure was kept at one bar and a cross flowvelocity of 1.0 m/s was maintained. The permeate (product water) whichflowed into either the lumen of the hollow fibers or the shell of themodule was collected and analyzed. Samples of retentate(oil) were alsocollected and analyzed for their COD content. After ultrafiltration, theCOD of the permeate(product water) was found to be about 280 mg/L. Thisis a reduction of more than 95% in COD levels and the COD level was muchlower than the generally accepted standards for water disposal which is600 mg/L.

A sustained 70 hour ultrafiltration run was performed using the samesetup to determine the fouling characteristics of the hollow fibermembrane by oil. In FIG. 2, no appreciable change in the permeation fluxof the membrane was observed in a prolonged operation of the samecross-flow ultrafiltration unit in a 70 hour run. This indicates that noappreciable fouling of the membrane had occurred and therefore nostoppage for maintenance was required.

1. A hollow fiber membrane made of a cellulose ester for the separationof oil from water, the membrane exhibiting a Molecular Weight Cut-Off(MWCO) of about 5,000 to 30,000, a COD removal of above 95 percent and alow oil fouling tendency.
 2. A hollow fiber membrane according to claim1 wherein the membrane has a pure water permeability of about 100 to 300L/m².h.bar.
 3. A hollow fiber membrane according to claim 1 wherein themembrane has a wall thickness of 200 to 500 microns.
 4. A hollow fibermembrane according to claim 1 wherein the cellulose ester is celluloseacetate.
 5. A process for making cellulose based hollow fibers for theseparation of oil from water which comprises the steps of: a. forming adoping solution of about 15-25 weight percent cellulose acetate polymer,about 60-81 weight percent of organic solvent and about 4-15 weightpercent non-solvent additive or additives; b. spinning the hollow fibersvia the phase inversion technique using a tube in orifice spinneret; andc. leaching hollow fibers of solvent and non-solvent additive(s) byimmersing in a water bath.
 6. A process according to claim 5 wherein themaking of the cellulose based hollow fibers is performed at atemperature of 10° C. to 30° C.
 7. A process according to claim 5wherein said organic solvent comprises: N-methyl-2-pyrrolidone,dimethylacetamide, acetone, dimethylsulfoxide, dimethylformamide anddioxan.
 8. A process according to claim 5 wherein said non-solventadditives comprises of: polyvinylpyrrolidone, or inorganic acids, ororganic acids, or inorganic salts or a mixture of all or some of thementioned compounds.
 9. A process according to claim 5 wherein said stepof spinning the hollow fibers via the phase inversion technique using atube in orifice spinneret further utilizes an external coagulant ofwater and an internal coagulant of water or a mixture of water and NMP,where the mixture has a NMP composition of 20-80 wt. % in water.
 10. Aprocess according to claim 9 wherein said step of spinning the hollowfibers via the phase inversion technique using a tube in orificespinneret further exposes the hollow fiber to an air gap of 0-50 cmbefore it reaches the external coagulant.
 11. A doping solution forspinning of cellulose acetate hollow fiber membranes for the separationof oil from water comprising of: 15-25 weight percent of celluloseacetate polymer, 60-81 weight percent of organic solvent and 4-15 weightpercent of non-solvent additive or additives.
 12. A doping solutionaccording to claim 11 wherein said non-solvent additive or additivescomprises of: polyvinylpyrrolidone, or inorganic acids, or organicacids, or inorganic salts or a mixture of all or some of the mentionedcompounds.
 13. A doping solution according to claim 11 wherein saiddoping solution is prepared at temperature of 10-30° C.